The present invention relates to methods for air disinfection by photocatalytic oxidation, and, in particular, to methods in which the air to be disinfected is provided at a relative humidity greater than about 40%, and then contacted with a photocatalyst in the presence of ultraviolet (UV) light so that at least a portion of any microorganisms in the air are destroyed by photocatalytic oxidation. The present invention also relates to air disinfection devices having a photocatalyst-coated surface illuminated by a UV light source and having means for contacting air with the photocatalyst-coated surface and means for maintaining the relative humidity of the air above about 40%, so that microorganisms in the air are destroyed by photocatalytic oxidation when the photocatalyst-coated surface is illuminated with UV light.
Americans spend 90% of their time indoors, and while indoors, are exposed to a variety of airborne contaminants such as volatile organic compounds (VOC's), radon and biological organisms. In a 1980 study, the Environmental Protective Agency (EPA) concluded that indoor air pollution posed a greater health risk than outdoor air pollution. Indoor air contamination is estimated to cause significant increases in medical costs and a decline in work productivity.
In building circulating systems, pollutant levels from individual sources may not pose a significant risk by themselves. However, many buildings have more than one source that contributes to indoor air pollution. Illnesses resulting from such indoor pollutants are sometimes known as the "sick building syndrome." The causes of indoor air pollution are unwanted particulate matter, unwanted chemical substances and microbial contaminants. In the first two cases, conventional technology can oftentimes provide a solution by filtration and adequate ventilation. The problem of volatile organic compounds (VOCs) and microbiological contamination creates a more serious obstacle.
Because so many Americans spend a great deal of time in offices and buildings with mechanical heating, cooling, and ventilating systems, such systems pose a risk of biological contamination. In recent years, biological problems in indoor environments have received considerable attention. The Legionnaires' disease outbreak in Philadelphia in 1976 is probably the most publicized case of illness caused by indoor pollutants.
Biological contamination includes bacteria, molds, and viruses. A contaminated central air system can become a breeding ground for biological contaminants and the forced air can distribute the contaminants throughout the building.
During the past three to four decades, significant changes have occurred in the construction and operation of buildings. The building envelope has become tighter, thus less outside air is being used in heating, ventilation and air conditioning (HVAC) systems. Microbial agents are not so readily diluted by mixing with fresh air. Energy conservation programs have resulted in buildings with moisture in the indoor environment, and that has facilitated the growth of certain types of microorganisms.
Because of neglected maintenance programs in some buildings, excessive dirt, a nutrient for microorganisms, accumulates in niches of the HVAC systems. This results in maladies referred to as building related illness, building sickness and sick building syndrome.
Several microbiological particle control techniques exist (including mechanical and electrostatic filters) that may be used as part of the building's forced air heating/cooling system to reduce indoor concentrations of respirable particles. Microbiological filters have been used for disinfection of air and other gases because of their low cost and ease of handling. These filters can be constructed to remove not only microorganisms but submicron particles as well. For efficient and economic operation of these filters, the aerosol content of the air to be filtered must be low. (Microorganisms, particles, or droplets of liquid dispersed in air are referred to as aerosols.) A disadvantage of such filters is that they do not permanently remove the contaminants, but just transfer them to another medium; that is, the filter. Clogging will cause high pressure drops in the duct.
One viable solution is to permanently remove the contaminant and produce nontoxic residue. UV disinfection has been widely used in the past to destroy biological contaminants and toxic chemicals.
Such UV treatment has worked well for disinfection but the indoor environment may also be contaminated with low level toxic chemicals such as formaldehyde, styrene, and toluene. Ultraviolet energy alone has proven ineffective in degrading these chemicals. For instance, U.S. Pat. No. 5,045,288 to Raupp and Dibble, and U.S. Pat. Nos. 4,892,712; 4,966,759; and 5,032,241 to Robertson and Henderson use UV to treat fluids and gases that contain pollutants.
One alternative that has gained much attention is photocatalytic oxidation, which involves the use of a photocatalyst such as TiO.sub.2 for the total destruction of hydrocarbons and microorganisms in water. Patel, Antibacterial Effect Of Catalyzed Radiation, Masters Thesis, University of Florida, Gainesville (1993) reports powdered TiO.sub.2 to be capable of killing Serratia marcescens after irradiation for 60-120 minutes in water. Saito et al., J. Photochem. Photobiol. B:Biol., 14, 369-79 (1992); Matsunaga, J. Antibact. Antifungic. Agents, 13, 211-20 (1985); Nagane et al., J. Dent. Res., 68, 1696-7 (1989) and Moriaka et al., Caries. Res., 22, 230-1 (1988), report TiO.sub.2 to be capable of killing E. coli and Lactobacillus acidophilus after aeration for 60-120 minutes in water. Wang et al., Proceedings of the First International Conference on TiO.sub.2 Photocatalytic Purification and Treatment of Water and Air (London, Ontario, Canada, Nov. 8-13, 1992) pp. 733-9; Wang et al., Proc. AWWA Conf. (San Antonio, Tex., 1993); Savat et al., J. Catalysis, 127, 167-77 (1991) and Anderson et al., Further Catalytic Purification of Water and Air, 1, 405-20 (1993) report the gas phase detoxification of trichloroethylene (TCE) and other organic contaminants. There remains a need for a method by which microorganisms may be removed from the air.